7 PCB surface treatment technologies
PCB surface treatment technology refers to the various methods used to modify or enhance the surface of a printed circuit board (PCB). These treatments are applied to the copper surfaces of the PCB to improve the adhesion of components, protect against oxidation, and facilitate soldering during the assembly process. There are several types of PCB surface treatment technologies available, including:
Organic solderability preservatives (OSP)
OSP seems simple, but its molecular composition is quite complex. This large organic molecule is dissolved in water and organic acid solution. The PCB is immersed in the solution. The exposed copper forms chemical bonds with these molecules, forming an OSP-copper combination deposition layer on the copper surface, the thickness can reach 0.10~0.60um.
The disadvantage is that once the OSP coating is too thick, it will affect welding. There is also no way to check whether the copper is fully protected. It should also be noted that PCBAs using conventional wave soldering and selective wave soldering processes are not allowed to use OSP surface treatment.
(1) Lead tin spraying
Lead tin spraying, also known as lead-tin plating or lead-tin coating, is a process used to apply a protective layer of lead and tin onto a surface. It involves spraying molten lead-tin alloy onto the desired object, which then solidifies into a thin, uniform coating. This coating provides corrosion resistance, improves solderability, and enhances the overall durability of the treated surface. Lead tin spraying is commonly used in industries such as electronics, automotive, and aerospace.
(2)Lead-free tin spraying
Lead-free tin spraying is an environmentally friendly process that does very little harm to the human body. It is also a process advocated at this stage. The lead content in lead-free tin does not exceed 0.5. Lead-free tin has a higher melting point, which makes the solder joint much stronger. In fact, lead-tin spraying and lead-free tin spraying are both processes, but the purity of the lead is different. Lead-free tin is more environmentally friendly and safer for the human body and the environment and is also a development trend in the future.
Immersion gold is a thick layer of nickel-gold alloy with good electrical properties wrapped on the copper surface, which can protect the PCB for a long time; in addition, it also has environmental tolerance that other surface treatment processes do not have. In addition, immersion gold can also prevent the dissolution of copper, which will be beneficial to lead-free assembly.
Its advantages are that it is not easy to oxidize, can be stored for a long time, and has a flat surface. It is suitable for welding fine gap pins and components with small solder joints. This method is the first choice for PCB boards with buttons (such as mobile phone boards). It can be reflowed many times without reducing its solderability. It can be used as the base material for COB (Chip On Board) wiring.
Its disadvantages are higher cost and poor welding strength. Because it uses an electroless nickel plating process, it is easy to cause black disk problems. The nickel layer oxidizes over time and is not reliable for the long term.
Chemical Immersion Silver (IMS)
The copper on the PCB requires separate cleaning and micro-etching pretreatment. In the silver solution, a Giovanni replacement is performed between the inert copper and the more inert silver.
The Giavanni effect is also known as the galvanic cell effect and galvanic corrosion. That is, two connected metals with different activities are dissolved and contacted with the electrolyte to cause a galvanic cell reaction. The more active metal atoms lose electrons and are oxidized (corroded).
In this process, washing and drying are critical to ensure that impurities are removed. If multiple immersions of silver are required, or the silver thickness exceeds 0.5um, the Giavanni effect will affect the copper and even cause an electrical open circuit.
The resistance of chemical immersion silver to creep corrosion is relatively poor. Creep corrosion refers to the phenomenon that PCB or PCBA is exposed to moisture and other gas environments and slowly corrodes crystals over a certain period of time, which may cause short circuit or open circuit problems.
Immersion Tin (IMT)
The immersion tin process is specially designed to chemically deposit tin metal plating on the copper surface to facilitate SMT and chip packaging. It is a new green and environmentally friendly process that replaces the Pb-Sn alloy plating process and has been widely used in electronic products ( Such as circuit boards, and electronic devices) and surface treatment of hardware, decorations, etc.
The method of chemical immersion tin is similar to that of chemical immersion silver, except that tin is an amphoteric metal and can react with both acids and alkalis. Therefore, after deposition, contact with strong acids and alkalis should be avoided.
Electroless Nickel Immersion Gold( ENIG ) and Electroless Nickel Electroless Palladium Immersion Gold (ENEPIG)
ENIG is the abbreviation of Electroless Nickel Immersion Gold. It consists of a thin layer of electroless nickel plating and immersion gold, involving a three-layer metal structure of CU/NI/Au. The process flow mainly includes: activated copper, electroless nickel plating, and chemical immersion gold. Gold is an ideal element for PCB outer layer coating because gold does not form oxides, is less affected by temperature and storage conditions, and is easy to solder. However, if the gold content exceeds 3% of the solder mass, the solder joint will become brittle, so the maximum thickness of the gold layer during welding is 0.3um.
ENEPIG is the abbreviation of Electroless Nickel Electroless Palladium Immersion Gold.
ENIG is mainly used for surface treatment of circuit boards to prevent the copper on the surface of the circuit board from being oxidized or corroded and is used for welding and contact applications, such as gold fingers on memory modules.
ENEPIG is mainly used for surface treatment of packaging substrates. During the nickel-palladium immersion gold process, by controlling the electroless palladium plating and immersion gold control on the nickel layer, precise deposition thickness and gold layer uniformity are obtained to achieve a good contact surface.
PCB hybrid surface treatment technology
PCB hybrid surface treatment technology refers to a method of enhancing the surface properties and functionality of a printed circuit board (PCB). It is a combination of different surface treatment techniques used to make the PCB more reliable, durable, and suitable for various applications.
Typically, PCB hybrid surface treatment involves the integration of multiple surface finishes, such as solder mask, copper plating, gold plating, immersion tin, or immersion silver. The combination of these treatments depends on the specific requirements of the PCB and the desired functionality. The purpose of using hybrid surface treatment is to improve solderability, corrosion resistance, electrical conductivity, and overall performance of the PCB.
All in all, when it comes to the type of surface finish we choose, we must choose the best type to accomplish numerous functions. Each type of surface treatment has its own advantages and disadvantages. There are some engineering tricks that can solve problems caused by shortcomings in surface finish. For example, there are some solutions to the shortcomings of OSP’s lower wetting power, such as changing board solderability plating or wave soldering alloys, increasing top surface preheating, etc. The key is that all possible factors must be considered to obtain ideal performance.